Crystal Report formula with datediff not working as expected

Question

We need a Crystal Report formula to display the number of seconds the oldest arriving call has been waiting.  Across multiple resources that can each have an "oldest call".  The database stores a datetime value for the time the oldest call arrived.  If there are no waiting calls, then this field is NULL.  (MSSQL database).  It seemed reasonable to implement this in a formula that 1) discovers the minimum of "oldest call" timestamps in the selected records, and 2) to use "datediff" to produce the difference (in seconds) between the "oldest call" timestamp and current time.
The first attempt at this relied on "implied" iteration that could be done within a formula.  Something like:
data: OLDESTARRIVALTIME
       null
       '2014-06-14 08:08:08.000'
       null
      '2014-06-14 08:07:55.000'
       null
whilereadingrecords;
datetimevar minArrival;
if isNull({SVCCLASSMEASURES_VW.OLDESTARRIVALTIME}) = False
               and minArrival < {SVCCLASSMEASURES_VW.OLDESTARRIVALTIME} then
     minArrival := {SVCCLASSMEASURES_VW.OLDESTARRIVALTIME};
DateDiff("s", minArrival, {SVCCLASSMEASURES_VW.UTCDATE})
We tried storing the values of OLDESTARRIVALTIME in an array.  We could see it iterating, but the values in the array only contained
the column value from the first record.
This was to solve the problem of screening null values and producing the minimum of the set.
Since that didn't work (and the web articles seemed to imply this would iterate over all the records, we tried another approach.  This
time we set NULL timestamps in the table to a timestamp far in the future, so that we could directly apply "minimum" to produce
the correct "begin" for datediff.
data: OLDESTARRIVALTIME
          '2030-01-01 00:00:00.000'
          '2014-06-14 08:08:08.000'
          '2030-01-01 00:00:00.000'
          '2014-06-14 08:07:55.000'
          '2030-01-01 00:00:00.000'
datetimevar minArrival = minimum({SVCCLASSMEASURES_VW.OLDESTARRIVALTIME});
datetimevar minUTC = minimum({SVCCLASSMEASURES_VW.UTCDATE});
if minArrival < minUTC then
     DateDiff("s", minArrival, minUTC)
else
     0
(minUTC would be current time in UTC)
So, to start things off, the last formula produces negative numbers!  They hover in negative seconds within a negative minute (-33, -45, etc.).
That's inconceivable, considering the test for minArrival < minUTC.  Both of the fields are "datetime".
As it turns out, in the near term, it's most important to get the second formula working.  And, of course, insights into getting the first
formula to work are welcome as well!
Have we run into some weird behavior of the "DateDiff" function?
Thanks!

Answer 1

If DateDiff is always returning a negative number you could try swapping the dates around in the call to DateDiff - it should look like this:
     DateDiff("s", minUTC, minArrival)    
Or you could use the Absolute Value of the calculation:
     Abs(DateDiff("s", minUTC, minArrival) )
As for your first formula, you need to initialize the variable to a value prior to using it in the comparison.  If you don't, its value is null and comparing it against a value won't work.  (See What is Null and Why is it Important for Crystal Reports | SAP BI BLOG for a blog that I wrote about working with nulls in Crystal.)
You should change your existing formula to something like this:
whilereadingrecords;
datetimevar minArrival;
if OnFirstRecord then minArrival := DateTime(2013, 1, 1, 0, 0, 0);
if not isNull({SVCCLASSMEASURES_VW.OLDESTARRIVALTIME})
               and minArrival > {SVCCLASSMEASURES_VW.OLDESTARRIVALTIME} then
     minArrival := {SVCCLASSMEASURES_VW.OLDESTARRIVALTIME};
Note how I changed the If statement.  Also, you want to replace minArrival with the field value if the field is less than the current value of minArrival - so you need to use ">" instead of "<" in the comparison.  Place this formula in the details section.  It will now show anything because of the semi-colon on the end.  This will ensure that it gets evaluated for every record.
Now, create a second formula that looks like this:
whileReadingRecords;
datetimevar minArrival;
DateDiff("s", minArrival, {SVCCLASSMEASURES_VW.UTCDATE})
Place this formula in a footer section - it will not work in a header section.  If you need it in a header section you might be able to take the "whilereadingrecords" off of both formulas and use the "Maximum()" summary function to get the correct value.
-Dell